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Bill Gurstelle is your MAKEcation counselor for the make-a-trebuchet Family Challenge. Build a trebuchet and post pictures tagged "MAKEcation" in the MAKE Flickr pool to enter to win a $100 Maker Shed gift certificate!
More in my series of posts on How to Build a Catapult. In this post, I discuss Catapults, Ballistas, Trebuchets and the Triggers, Releases, and Latches that operate them.
I receive quite a few inquiries about making catapults and trebuchets. And among the most frequent questions is: how do I build the trigger? It's an important question.
Catapults are fun and educational to build and operate. When building a catapult, trebuchet, ballista, mangonel, or any of the hundred names by which such hurling machines are known, you'll find out that the mechanism that releases the throwing arm is often the most complicated part of the machine.
You may build your own release, which is tricky because you need to design the latch so that it will release reliably under full load. Actually, you can get pretty creative about triggers.
But considering how cheap and easy it is to buy one, it might make more sense to use one of the commercial solutions below, and spend your time working on the other aspects of the machine.I've spent time researching the best triggers and releases for small catapults. There are three good, off-the-shelf solutions: The archer's arrow release, the sailor's pelican hook, and the horse trainer's panic snap.
Archer's Arrow Release:
This is the premier solution. Works dependably, quickly and very smoothly. It costs a bit more, but of all the catapult releases I've tried, I like this one the best. Find it at a sporting goods store with an archery department, or find it online.
Sailor's Pelican Hook:
This is another excellent hurling machine trigger. Its normal use is in sailing, where it is used to securely hold and release lines and ropes. Basically a pelican hook is a hinged hook that can be quickly secured or released by a sliding ring. It is quite a bit less expensive than an arrow release, but it holds securely under load. The downside is that they can be awkward to release sometimes. Find it at a boating store or online.
Horse Trainer's Panic Snap:
A panic snap is a mechanism often used between a lead and a horse's harness. They are decent catapult triggers because they can be disconnected under load. A panic snap is specially built so that the latching mechanism is separate from the load bearing structure. Just pull back on the latch and the load releases. Very inexpensive, but not as smooth or dependable as the arrow release. Find it at a tack shop or online.
More:
I built a tennis ball chunking trebuchet last fall, and my trigger was very simple. I scaled up the trigger from a table-top trebuchet kit from trebuchet.com --
Two eye bolts on the catapult frame, and one on the arm. The eyes all line up so you can put a pin through them when the arm is down. The pin was a long 1/4 inch bolt, attached to a lanyard, and simply pulled out to fire.
This won't work with catapults with very heavy counterweights, but for mine (50-75 lbs), it did the trick.
Since the catapult was used by Boy Scouts, I also included a redundant safety of a second hold-down rope that was removed just prior to firing. That way if safety broke down, and a boy removed the pin, the arm would not release.
Then, when the weight box broke, we rigged the catapult as a traction trebuchet. With four 11-year-old boys pulling ropes attached to the counterweight attachment point, we got the same range as 75 lbs of counterweight.
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To me the picture of the Horse Trainer's Panic Snap looks like a faceless and armless fellow sitting on the rim of a wheel.
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Watch Bill put these connectors to the test here:
http://blog.makezine.com/archive/2009/02/make_television_episode_7_urban_pro.html
or check local listings on your PBS station. - Make: TV
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Here is the high dollar load release option:
http://www.seacatch.com/
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A car seatbelt latch is designed to take a lot of pulling force, and release under load.
The "load" is figured to be the slumped-over figure of a driver or passenger, but it's still a load.
Older, all-metal ones are better.
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